Here's my basic list that I think would make it lots of fun!

General:
-Corals
-Anemone
-Live rock
-Sump/protein skimmer for filtration
- Special lighting for corals

Small fish:
-Jellyfish (species only tank)
- Sand sifting starfish
- Brittle starfish
- Hermit crabs, emerald crabs
- Peppermint shrimp!
- Mandarin goby
- Six-line wrasse

Medium fish:
- dwarf angelfish (such as the flame angel, bicolor, or lemon peel)
- Pajama cardinal
- Damselfish/chromis

Large fish:
- Hippo/blue tang
- Yellow-eyed Kole
- Porcupine fish
- Clown trigger
- Picasso trigger
- Lionfish

I personally am more of a fan of freshwater species, but it would definitely be cool to have a saltwater version. You would need to keep in mind reef-safe versus not species, and I think it would also be cool to be able to stack rocks (both for the saltwater and freshwater versions of the game).

Don't have much to say about stuff other than fish but I'd really love a leopard moray or a snowflake eel as well as some species of catshark/dogfish

i would like to see a seahorse. I love it! ❤ Corals would be nice too.

I agree with what Komorebi posted above but would also add Clown Fish, a long time fan favorite, that would live in the anemones. Perhaps special tanks to raise brine shrimp to feed to the fish and supplies to feed the anemone and corals. Test kits for both salt and freshwater. Hydrometer and Refractometer to test the salt content of the water and salt to start and maintain the water quality. Snails and crabs for both types too.

Hello!
Nice of you, to ask!
I would want to see Yellyfishes in their various kinds. And the little fishes, what are called out in the Pedia from the fishes that already exist, to combine them. That would be nice.
Gigantic tanks und lots of missions. I love missions and experiments through them! More, than build aquariums on my own.
Maybe a Fish Museum Mission, that you can look at whenever you like after? That would have style, if it is possible for the creatures of the sea. So you can implement lots of more like krokodiles. Just for this Quest-task.
And maybe lots of verious kinds of algae would be nice. They can be really pritty!

In the saltwater aquarium world there are some different equipment you should look to add. Although heaters are the same, there is a larger need for water movement. Water circulation is very important in a coral tank so having different sized pumps is important. Also Saltwater aquariums use protein skimmers to aid in cleaning the water. Here are some other ideas:

-crushed coral substrate
-sump tanks with beneficial bacteria and algae
-lots of fun crabs to add like decorator crab, sally lightfoot, or porcelain crab
-Dosing systems to dose calcium and other additives

I understand you need money but I would prefer a well-programmed freshwater aquarium first.
example corydoras needs a hidden level max 10cm above the bottom.
And similar things that can be easily programmed are more.

Hey there! I'd really like it if you added these to the Salt water DLC:

Sumps and other external filtration options, biotics, skimmers as well as UV filtration are REALLY important for Salt water aquariums
More lighting options will be needed.
More variations in substrate. Not only single type but the ability to mix your own
ways to attach anemones and clams to existing hardscapes


The most important addition:
modding capability.

PS I'm also willing to provide free English proof reading for your scripts.

It'd be great to be able to place objects and things on top of one another so you could properly construct a reef!

I have to agree with others who have said I'd prefer if you worked more of the kinks out of the freshwater before working on a DLC though - things like fish motion, stacking hardscape and rotating it in other planes are pretty important!

Sharks
Stingrays
Queen Angels
Seahorses
Clownfish
Triggers
Turtles
Lionfish
Boxfish
Harlequin Tusk
Octopus
Parrotfish
French Angel
Majestic Angel
Emporer Angel
flame angel
Gobies
Heiochus Butterfly
Banded Catshark
Koran angel
Blue eye cardinal
Crabs
Rabbitfish
Surgeons
Tangs
Wrasse
Star fish
Live coral
Lemon peel ( Centropyge Flavissima)
Foxface
Damsels
Butterfly fish
Angel fish
Cowfish
Pipe fish
Groupers
Eels
Porcupine Puffers
Basslets
Marmalade Shark ( Atelomycterus Marmoratus)
ability to make larger tanks like you can see at sealife centres
more fish like the ones that you can see on the Tanked tv show

https://www.liveaquaria.com/product/aquarium-fish-supplies.cfm?c=15

everything in it lol

Gotta have coral. 100%

Oh, my. Seahorses and sea dragons, nudibranches, maxima and derasa clams, stars, sponges, anemones, urchins, corals, octopodes and jellies... I'd even say sneak a trilobite or coelacanth in there and see if anyone notices. :D

For now, we're focused on fishes but I'd love to add other organisms as well one day :)

Please add jellyfish :) that is my biggest wish! Even if they are in a tank alone, they are so beautiful to watch swim ♡